Photodynamic therapy applying a photosensitive substance to a lesion being difficult to identify, such as a tiny tumor tissue, has been drawing attention in the field of surgical operations using an operation microscope. This photosensitive substance has characteristics that the substance has affinity for a lesion such as a tumor, and is accumulated on the lesion when administered to a patient. The photodynamic therapy utilizes these characteristics and conducts a treatment by irradiating an affected part with excitation light and observing fluorescence emitted from the excited photosensitive substance.
PTL 1 discloses an operation microscope equipped with a light source for excitation light, which enables fluorescence observation using the excitation light in addition to ordinary observation using natural light.